The present invention relates to an improved apparatus, such as a nebulizer apparatus, for discharging liquid, Nebulizem, or atomizers, are devices that generate a fine spray or aerosol, usually of liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or a suspended particulate or colloidal phamaceutical agent for administration to a subject by inhalation. Such inhalation treatment is highly effective for conditions affecting the subject""s respiratory organs. Further, since the lungs are close to the heart and the blood circulatory system of the body, drug administration by inhalation provides an effective and rapid delivery system to all organs of the body. Other applications include dispensing insecticides, paint, deodorants, water for humidification, etc. Other apparatuses which may incorporate the present invention include printers in which ink is discharged onto paper.
When dispensing a pharmaceutical agent, in many cases, a nebulizer is placed directly in the mouth or nose of the subject so that the spray can be entrained in the respiratory gases inhaled during normal, spontaneous breathing of the subject. In other cases, the subject breathes with the aid of a respiratory ventilator. A typical ventilator has a breathing circuit comprising an inhalation limb and an exhalation limb connected to two arms of an Y-connector. The third arm of the Y-connector is connected, via a patient limb, to a mouthpiece, mask or endotracheal tube for the subject. The ventilator provides a complete or partial supply of respiratory gases to the subject through the inhalation limb during inhalation. The contraction of the subject""s lungs discharges gas through the exhalation limb during exhalation. When a nebulizer is employed in conjunction with a ventilator, it is typically placed in the patient limb but can also be placed in the inhalation limb.
Nebulizers currently in use for ventilator applications generate the spray either pneumatically or by means of ultrasonic vibrations. Pneumatic nebulizers are typically used with a liquid, such as an aqueous drug solution. High pressure driving gas is conducted through a nozzle to draw the drug from a drug supply for the nebulizer. The drug is discharged against a baffle or other means in a gas space of the nebulizer, breaking the liquid into a fine spray. The gas space is in fluid communication with the inhaled gas pathway of the breathing circuit so that the gas flow expelled from the nozzle along with the nebulized drug is conducted to the breathing circuit and ultimately to the subject.
Disadvantages in the use of pneumatic nebulizers include the following. If the nebulizer adds a significant quantity of gas, for example, up to five liters/minute, into the breathing circuit, the breathing gas composition may be affected. Due to passage of the driving gas through the nozzle, impingement of the drug on the baffle, etc., pneumatic nebulizers are noisy. Also, controlling the commencing and stopping of a drug agent spray is difficult and not very accurate, resulting in wastage of the drug.
The foregoing shortcomings of pneumatic nebulizer have led to the use of ultrasonic nebulizers employing a vibrating element, such as a piezoelectric crystal. Breathing gas composition and the on-off operation are easier to control with such nebulizers than in a pneumatic nebulizer. However, ultrasonic devices may require a large, bulky electrical power supply to power the ultrasonic vibrator and may not be able to nebulize colloidal or particulate suspensions.
In one type of ultrasonic nebulizer, the fine spray is produced dropping the liquid on, or otherwise applying it to, the vibrating element. See U.S. Pat. No. 5,443,059. U.S. Pat. No. 3,812,854 describes another type of nebulizer, for use in inhalation therapy, in which the spray is generated on the front surface of a vibrating, porous body. The pores of the body form a network of passages that enable the liquid to flow through the body. The liquid to be nebulized is supplied under pressure to the pores on the rear surface of the body, and forced through the pores to the front surface of the porous body where it is discharged as a spray. U.S. Pat. No. 5,487,378 describes a nebulizer in which the aerosol is formed using a mesh plate instead of a porous solid body. The mesh plate has a plurality of orifices for the liquid. The liquid or the nozzle assembly is vibrated ultrasonically by a piezoelectric element to nebulize a dose of liquid as it passes through the mesh plate. The supply of each dose through the nebulizer is sensed by a dose gauge.
A specific difficulty with nebulizers in which the liquid or the orifice assembly is vibrated ultrasonically, as by a piezoelectric element, to nebulize the liquid is control of the supply of liquid to the nebulizer so that the right amount necessary for proper operation is present in the nebulizer.
U.S. Pat. Nos. 5, 518,179 and 5,299,739 describe nebulizers in which capillary feed is used to supply liquid to the vibrating element. A further alternative for liquid supply is achieved by condensing a liquid vapor on one face of the membrane, the liquid thus condensed being dispensed in droplet form. See U.S. Pat. No. 5,518,179.
U.S. Pat. No. 5,938,117 describes an apparatus for dispensing liquids as an atomized spray and having a fluid supply system that transports fluid to an apertured oscillating surface. The fluid supply system is connected to an electronic flow control valve. The valve is connected to an electronic circuit. In the event of excessive delivery of liquid, the oscillation amplitude decreases and the current draw by the piezoelectric element decreases. A current sensor senses the reduced current draw and transmits an overflow signal to the flow control valve to reduce the delivery rate of liquid to the surface until the amount of fluid returns to a normal level.
An object of the present invention is to provide an improved device and method for accurately controlling the supply of liquid in an apparatus discharging liquid. One such apparatus can comprise a nebulizer in which the accurate liquid supply enables the liquid to efficiently transformed into an aerosol. The invention may be employed with other types of apparatuses, such as those discharging ink for printing purposes.
The invention is particularly suited for use with a nebulizer employing an ultrasonically vibrating element but can also be extended to other type of nebulizers in which proper functioning and efficiency are dependent on control of the liquid into the nebulizer.
The above objects are obtained by an improved device and method for measuring the amount of liquid to be discharged that is present in the apparatus discharging the liquid. To this end, in a typical embodiment of the invention, a nebulizer includes a first member having holes through which the liquid passes to be nebulized. A second member is spaced from the first member so that a volume is defined in the nebulizer by the area of mutual overlap of the first and second members and the amount of spacing between them in the area of overlap. The first and second members have electrically conductive properties, as by being formed of conductive material or having an electrically conductive coating. The first and second members are electrically isolated from each other. The liquid to be nebulized is provided into the volume between the first and second members and circuitry is coupled to the first and second members to measure the capacitance between the members. The capacitance between the members indicates the amount of liquid in the volume defined in the nebulizer. A vibrator, such as a piezoelectric element, vibrates the liquid, as by bowing one of the first and second members, to carry out the nebulization of the liquid. The capacitance measuring circuit may be coupled to a liquid supply to cause the latter to, preferably intermittently, provide additional liquid to the volume as the nebulization of the liquid proceeds.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.